1. Field of the Invention
The present invention relates to an optical disc drive for reading and/or writing data from/on a disklike information storage medium (which will be referred to herein as an “optical disc”). More particularly, the present invention relates to an optical disc drive that can recognize the type of the given optical disc as one of multiple different types.
2. Description of the Related Art
Recently, various types of optical discs, including CDs (compact discs), DVDs (digital versatile discs) and BDs (Blu-ray discs), have become immensely popular.
The characteristics of these optical discs change from one type to another. Examples of those varying characteristics include a reflectance (i.e., the ratio of the intensity of light reflected from an optical disc to that of the light incident on that optical disc) characteristic, a wobble characteristic (i.e., the periodic spiral wobbled pattern that has been engraved on the surface of the disc), and a focus error signal (i.e., a control signal for use to control the position of an objective lens perpendicularly to an information storage layer such that the focal point of a light beam is always located right on the information storage layer) characteristic.
To read and write information from/on various types of optical discs, a so-called “disc type recognition” process for recognizing the type of the optical disc that is currently loaded in the optical disc drive needs to be carried out.
The disc type recognition can get done by utilizing the difference in characteristic between those optical discs. For example, the optical disc drive may recognize the type of the given optical disc by measuring the reflectance, the wobble period and/or the amplitude of the focus error signal of the optical disc as described above.
Among those various optical discs, a BD, for example, has a spherical aberration problem. It is known that once a spherical aberration has occurred, the quality of the read or written data deteriorates. Thus, an optical disc drive compliant with the same BD standard has a mechanism for correcting a spherical aberration.
FIG. 1 illustrates a state in which the spherical aberration has occurred. As used herein, the “spherical aberration” refers to a phenomenon that the focal point of a light beam that has passed through a center portion of an objective lens 101 as indicated by the dashed lines shifts along the optical axis from that of a light beam that has passed through a peripheral portion of the objective lens 101 as indicated by the solid line. It should be noted that the “spherical aberration” sometimes means the magnitude of shift between the respective focal points of the light beams as indicated by the dashed and solid lines in FIG. 1, not the phenomenon itself.
The magnitude of the spherical aberration changes due to various factors. Examples of such factors include the wavelength of the light beam, the numerical aperture (NA) of the objective lens, and the depth or distance from the surface of the optical disc on which the light beam is incident to the information storage layer (which will be referred to herein as the “depth of the information storage layer as measured from the surface”). Even if optical discs have been made compliant with the same BD standard, the depth of the information storage layer as measured from the surface may slightly vary (within the range tolerated by the standard) from one disc to another, or the optical discs may have a very small tilt angle with respect to the optical axis. Even so, the magnitude of the spherical aberration still changes significantly. That is why when information needs to be read or written from/on an optical disc in which the quality of the read or written data is easily affected by the spherical aberration, the optical disc drive should perform a spherical aberration correction process.
For example, the optical disc drive disclosed in Japanese Patent Application Laid-Open Publication No. 2007-122850 includes a spherical aberration correcting mechanism (or correcting section) between a light source that emits a light beam and an objective lens that converges the light beam. The spherical aberration correcting section is controlled differently according to the type of the given optical disc, thereby minimizing the magnitude of the spherical aberration (i.e., optimizing the magnitude of spherical aberration correction) and getting the correction process done. Furthermore, the optical disc drive disclosed in that patent document detects the ratio of variation in signal level between multiple spherical aberration states by changing the settings of the spherical aberration correcting section, thereby carrying out the disc type recognition process without being affected by any variation in distance from the disc surface to the information storage layer or by the recording state of the optical disc.
To determine whether or not the given optical disc is a type that would produce spherical aberration and thereby ensure good signal quality, an optical disc drive including such a spherical aberration correcting mechanism as the one disclosed in Japanese Patent Application Laid-Open Publication No. 2007-122850 as a piece of hardware has been used in the prior art. However, the addition of such a mechanism would cause an increase in the manufacturing cost of the optical disc drive. DVD and CD optical systems for optical disc drives, among other things, recently require cost-reduced mechanisms in most cases. That is why it is not beneficial to add such an expensive mechanism that would increase the overall cost.
The specifications of those various types of optical discs have been defined one after another compliant with their associated standards. Meanwhile, a non-standardized optical disc called “Dual Disc” has also been put on the market and has already gained some popularity. Thus, measures should also be found to play such optical discs properly, too.
Specifically, a dual disc is a read-only optical disc including an information storage layer compliant with the DVD standard (which will be referred to herein as a “DVD side”) and another information storage layer, on which data of a compact disc is recorded, (which will be referred to herein as a “CD side”). Such a dual disc is fabricated by bonding a substrate with the DVD side and a substrate with the CD side together.
FIG. 2A illustrates how to read data from a CD compliant with the CD standard (which will be referred to herein as a “normal CD”). On the other hand, FIG. 2B illustrates how to read data from the CD side of a dual disc 201.
In the normal CD 200 shown in FIG. 2A, the light beam that has passed through an objective lens 101 is focused on an information storage layer 203. The distance from the surface of the CD 200 to the information storage layer 203 is approximately 1.1 mm and the in-focus state defined by the standard is realized.
On the other hand, in the dual disc shown in FIG. 2B, the light beam that has passed through the objective lens 101 is not focused on the CD side 202. This is because the distance from the disc surface to the CD side 202 is approximately 0.9 mm, which is shorter than that of the normal CD 200 and is not compliant with the CD standard.
That is why if the user tried to read data from the CD side 202 of the dual disc 201 without changing the settings for the normal CD 200, then the light beam would not be focused on the information storage layer 202 and spherical aberration would be produced, thus deteriorating the resultant signal quality. Thus, it should be difficult to read the information as intended.
Furthermore, it is also known that due to the difference in the depth of the information storage layer as measured from the disc surface, not just the spherical aberration but also astigmatism and coma aberration are produced as well and also constitute factors that would cause the degradation of signal quality.
To guarantee that data can be read accurately enough even from such an optical disc, of which the information storage layer is located at a different depth from the standardized one as measured from the disc surface and which will produce various types of aberrations, some measures need to be taken to improve the signal quality. In that case, the overall cost of the optical disc drive needs to be cut down without using any mechanism that will increase the cost significantly just like the one disclosed in the patent document mentioned above.
In order to overcome the problems described above, the present invention has an object of providing an optical disc drive that can determine whether or not the given optical disc is a type to produce a spherical aberration, and that can ensure high signal quality without using such an expensive and bulky mechanism for correcting the spherical aberration.